Lei Deng

Size Effect on Flow Behavior of a Zr55Al10Ni5Cu30 Bulk Metallic Glass in Supercooled Liquid State

Size effect on the flow behavior of Zr55Al10Ni5Cu30 bulk metallic glass in its supercooled liquid state was investigated by compression tests with specimen diameters varying from 1 to 3xa0mm. It was found that the smaller the specimen, the higher flow stress exhibits. Strain gradient theory considering friction effect is validated to be suitable to rationalize this size effect. The more geometrical-necessary flow sites needed to be created in smaller specimens, the higher stress it may result in. Considering the efficiency of power dissipation and instability condition, processing maps of different specimens were constructed. With the specimen size decreasing, the processing condition corresponding to low temperature or high strain rate becomes disadvantageous to the thermoplastic forming of Zr55Al10Ni5Cu30, which is closely related to the local stress concentration and strain gradient induced by friction.

Optimization of gear blank preforms based on a new R-GPLVM model utilizing GA-ELM

A novel R-GPLVM is proposed to screen out critical dimensions of the preform.A newly GA-ELM framework seamlessly integrated with R-GPLVM is proposed.Discussions demonstrate that Gaussian kernel function has the higher accuracy.The relevant parameters of ELM are optimized with the improved performance.Engineering applications and FEM validate the feasibility of the proposed method. The determination of the key dimensions of gear blank preforms with complicated geometries is a highly nonlinear optimization task. To determine critical design dimensions, we propose a novel and efficient dimensionality reduction (DR) model that adapts Gaussian process regression (GPR) to construct a topological constraint between the design latent variables (LVs) and the regression space. This procedure is termed the regression-constrained Gaussian process latent variables model (R-GPLVM), which overcomes GPLVMs drawback of ignoring the regression constrains. To determine the appropriate sub-manifolds of the high-dimensional sample space, we combine the maximum a posteriori method with the scaled conjugate gradient (SCG) algorithm. This procedure can estimate the coordinates of preform samples in the space of LVs. Numerical experiments reveal that the R-GPLVM outperforms the pure GPR in various dimensional spaces, when the proper hyper-parameters and kernel functions are solved for. Results using an extreme learning model (ELM) obtain a better prediction precision than the back propagation method (BP), when the dimensions are reduced to seven and a Gaussian kernel function is adopted. After the seven key variables are screened out, the ELM model will be constructed with realistic inputs and obtains improved prediction accuracy. However, since the ELM has a problem with validity of the prediction, a genetic algorithm (GA) is exploited to optimize the connection parameters between each network layer to improve the reliability and generalization. In terms of prediction accuracy for testing datasets, GA has a better performance compared to the differential evolution (DE) approach, which motivates the choice to use the genetic algorithm-extreme learning model (GA-ELM). Moreover, GA-ELM is employed to measure the aforementioned DR using engineering criteria. In the end, to obtain the optimal geometry, a parallel selection method of multi-objective optimization is proposed to obtain the Pareto-optimal solution, while the maximum finisher forming force (MFFF) and the maximum finisher die stress (MFDS) are both minimized. Comparative analysis with other numerical models including finite element model (FEM) simulation is conducted using the GA optimized preform. Results show that the values of MFFF and MFDS predicted by GA-ELM and R-GPLVM agree well with the experimental results, which validates the feasibility of our proposed methods.

A size-dependent constitutive model of bulk metallic glasses in the supercooled liquid region

Size effect is of great importance in micro forming processes. In this paper, micro cylinder compression was conducted to investigate the deformation behavior of bulk metallic glasses (BMGs) in supercooled liquid region with different deformation variables including sample size, temperature and strain rate. It was found that the elastic and plastic behaviors of BMGs have a strong dependence on the sample size. The free volume and defect concentration were introduced to explain the size effect. In order to demonstrate the influence of deformation variables on steady stress, elastic modulus and overshoot phenomenon, four size-dependent factors were proposed to construct a size-dependent constitutive model based on the Maxwell-pulse type model previously presented by the authors according to viscosity theory and free volume model. The proposed constitutive model was then adopted in finite element method simulations, and validated by comparing the micro cylinder compression and micro double cup extrusion experimental data with the numerical results. Furthermore, the model provides a new approach to understanding the size-dependent plastic deformation behavior of BMGs.

An Approach to Optimize Size Parameters of Forging by Combining Hot-Processing Map and FEM

The size parameters of 6061 aluminum alloy rib-web forging were optimized by using hot-processing map and finite element method (FEM) based on high-temperature compression data. The results show that the stress level of the alloy can be represented by a Zener-Holloman parameter in a hyperbolic sine-type equation with the hot deformation activation energy of 343.7xa0kJ/mol. Dynamic recovery and dynamic recrystallization concurrently preceded during high-temperature deformation of the alloy. Optimal hot-processing parameters for the alloy corresponding to the peak value of 0.42 are 753xa0K and 0.001xa0s−1. The instability domain occurs at deformation temperature lower than 653xa0K. FEM is an available method to validate hot-processing map in actual manufacture by analyzing the effect of corner radius, rib width, and web thickness on workability of rib-web forging of the alloy. Size parameters of die forgings can be optimized conveniently by combining hot-processing map and FEM.

A Study of Hole Flanging-Upsetting Process

A combined process of hole flanging and flange upsetting was proposed. Both elastic-plastic FEM and experiments were employed to analyze the process. A 2mm thick 08AL sheet with 120mm outer diameter and 24.6mm center hole diameter was used as the blank. The effect of the Rf and the Rc values on the flanging quality were analyzed, where Rf was defined as the ratio of the die fillet size to the workpiece thickness t and Rcwas defined as the ratio of the clearance C to the workpiece thickness t. Also the effect of Rf and Rc on upsetting ratio Ru which was defined as the ratio of thickness before and after upsetting were studied. The finite element results were validated by experimental results. Also a 2mm thick flange without thinning and defects was gained in a reasonable range of Rc and a certain value of Rf.

Hot Deformation Behavior and Processing Maps of 7050 Aluminum Alloy

The hot deformation behavior of 7050aluminum alloy was investigated by hot compression tests in the temperature range of 573-773K and the strain rate ranging from 0.001s-1 to 10 s-1.The flow curves showed that the flow stresses increase with the increase of strain rate or the decrease of temperature.In order to determine the optimal processing conditions, hot processing maps were established based on experimental data and Dynamic Materials Model. The processing maps indicate that instability occur at low temperature and high strain rate. The optimum hot working region is the domain in the temperature range of 673-723K and strain rate range of 0.001-0.01 s-1,where typical recrystallization was observed in the optical microstructures.

Microstructure evolution and modeling of 2024 aluminum alloy sheets during hot deformation under different stress states

In this work, specimens of the 2024 aluminum alloy sheet were compressed and stretched along the original rolling direction at elevated temperatures. The microstructure evolution was investigated by characterizing the metallographic structures via electron backscattered diffraction technology before and after deformation. It was found that while recrystallization occurred in the compressed specimens, it was not observed to the same extent in the stretched specimens. This difference in the grain morphology has been attributed to the different movement behaviors of the grain boundaries, i.e., their significant migration in the compression deformation and the transformation from low-angle to high-angle boundaries observed mainly during tension deformation. The empirical model, which can describe the grain size evolution during compression, is not suitable in the case of tension, and therefore, a new model which ignores the detailed recrystallization process has been proposed. This model provides a description of the grain size change during hot deformation and can be used to predict the grain size in the plastic deformation process.

Microstructure and Texture Evolution of 2024 Aluminum Alloy Sheet under Different Loading Conditions

To study microstructure and texture evolution of 2024 aluminum alloy sheet under different loading conditions, thermal tensile and compression experiments of 2024 aluminum alloy rolled sheets were carried out at temperatures ranging from 300 °C to 450 °C and under strain rates ranging from 0.001 s-1 to 0.1 s-1. During tensile deformation, the HABs of original grains are directly elongated until abruption. DRX process occurs during compression. Dislocations appear during deformation, migrate and accumulate into LABs, and then rotate into HABs to form new grain.The three-dimensional orientation distribution functions (ODFs) in different stress states were measured, with related texture types and distribution laws compared. According to ODFs with a constant φ2, the deformation texture of {011} <100>Goss texture is gradually strengthened during thermal tension at high temperature and low strain rate (450°C/0.001s-1). The deformation texture of {011} <100>Goss texture is weakened with the strain increasing. Furthermore, the increase of deformation temperature or the decrease of strain rate slows down the weakening process of {011} <100> Goss texture, which is attributed to the recrystallization behavior during tensile deformation. Besides, since the recrystallization process proceeds more completely during hot compression, it produces a quasi-random texture.

Study on Multi-Step Spinning Process for Disk-Like Part with Thickened Rim

This paper studies the process of a five-step spinning to thicken the edge of the disc-like part. By using finite element simulation and experiment, the sectional shape and flow-line distribution of the rim were studied. The results showed that the flow lines of the cross-section of the formed part are distributed along the shape of the part. The disc-like part with thickened rim can be well formed by a multi-step spinning process. A large bottom radian of roller groove can lead a folding in the first step and a reduced r can overcome the folding. An over-small angle αin step 2 will lead a pit defect, it can be solved by increasing the angle.

Stamping-Forging Processing of Sheet Metal Parts

SFP is a combined metal forming technology of stamping and forging for sheet metal parts. In an SFP, generally, stamping or drawing is used to form the spatial shape of the part first, and followed by a bulk forming employed to form the local thickened feature. It is suitable for making sheet metal parts which have local thickened feature, such as single or double layers cup parts with thickened inner or outer wall, disc-like parts with thickened rim, etc.